1. Field of the Invention
The ubiquitous beach umbrella has long been an essential part of a day's stay at the beach. This is even more true with the current worries about the deleterious effects of exposure to ultraviolet light. Traditionally, the umbrella was anchored by simply driving the shaft into the sand. For this purpose, the shaft might be equipped with a sharp tip to make insertion easier, but a sharply pointed shaft represents something of a safety hazard. Furthermore, if the substrate is soft, like dry sand, a shaft that is merely pushed into the ground will probably not be inserted deeply enough to withstand the force of prevailing wind on the attached umbrella. On the other hand, if the substrate is more resistant, like wet sand or gravel, it will be very difficult to drive the shaft no a sufficient depth to give adequate support.
2. Description of Related Art
The prior art has attempted to solve this problem by providing a number of different screw-type devices that are attached to the umbrella shaft.
U.S. Pat. No. 2,103,948 to Jones shows a tubular, pointed socket for receiving the end of the umbrella shaft. Around this is a shallow, helical thread. Such a shallow thread provides neither adequate driving force nor sufficient anchoring ability while still having a dangerous point.
U.S. Pat. No. 2,441,109 to Carlson shows a shaft socket which has a pointed "corkscrew" at its lower end and handles to help screw the device into the ground. This unit appears to have the same drawbacks of the prior art, plus the additional complexity of handles.
U.S. Pat. No. 4,832,304 to Morgulis is similar, but the handles are pivotable and form part of the clamping arrangement.
U.S. Pat. No. 5,046,699 to Perreault et al. has fixed handles and a dangerously pointed screw member.
U.S. Pat. No. 2,628,797 uses an eccentric crank, rather like a brace and bit, to drive a twisted screw in a form similar to a wood drill bit.
These inventions all suffer from screw structures that have insufficient surface area on the screw threads to produce adequate force to draw the device deeply into the sand. U.S. Pat. No. 5,122,014 to Genfan attempts to remedy these shortcomings by providing larger driving blades which provide more pull and better anchoring. The device can penetrate the sand despite the large size of the blades because sand is scooped into the hollow shaft. However, such a device is ineffective in gravel or other resistant materials because excessive torque is required to turn the shaft. U.S. Pat. No. 5,088,681 to Procaccianti et al. approaches the problem by providing a broad helical flight attached to a sharply pointed elongate member. The flight would provide sufficient driving force and, area for anchoring, but would require excessive torque in a hard substrate as the broad helical flight must be forced through the resistant material.
Apparently the prior art is not fully effective. Sharply pointed stakes present a hazard to the user. Most of the prior art devices lack spiral threads of sufficient lateral extent to draw the device into a resistant substrate and to provide sufficient anchoring against prevailing winds. Those devices that do have threads of sufficient lateral extent are extremely hard to turn in resistant substrates, even with the additional complexity of special handles for the application of force.